Researchers in Norway and Germany have grown aluminum gallium nitride (AlGaN) nanocone arrays on silicon using a graphene mask.
[A. Mazid Munshi et al, Appl. Phys. Lett., vol113, p263102, 2018]
The research team of CrayoNano AS of Norway, the Institute of Optics of the Max Planck Institute of Germany, the Heilholz Berlin Energy and Materials Center, the Norwegian University of Science and Technology (NTNU) and the Norwegian Institute of Science and Technology (SINTEF) saw the material in the UV Potential applications of (UV) light-emitting diodes, photodetectors and lasers. They hope that this material will overcome the problem of planar Group III nitride structures that are caused by defects in lattice and thermal expansion mismatch. Graphene can achieve quasi-Van der Waals extension and avoid problems caused by hanging chemical bonds.
Ultraviolet light below 400 nm is used for water and air curing, sensing, food processing and microbial disinfection. In terms of sterilization applications, the wavelength limit will be more stringent. For example, in order to be able to disrupt the DNA structure, deep UV of less than ~250 nm is required. These very short wavelengths require the use of AlGaN alloy materials with a high aluminum fraction. Until these problems are resolved, the system will continue to rely on short-lived, toxic, bulky mercury lamps.
The single layer graphene was grown on copper foil by chemical vapor deposition (CVD) and then transferred onto a 2 inch silicon (Si) substrate with a native oxide layer. A hole pattern is formed in graphene by electron beam lithography (Fig. 1).
Figure 1: (a) Schematic representation of a hole mask and AlGaN nanopyramid growth on graphene. (b) Top view (c) Scanning electron microscope (SEM) image: The AlGaN nano-tapered array has a square pattern of 1 μm pitch, showing good uniformity and very high nucleation yield. Illustration (b): Scale bar 400 nm. (d) Top view SEM image of an AlGaN nano-conical array having a 2 μm pitch triangular pattern. Illustration (d): scale bar 2 μm.
AlGaN nanocones were grown using metal organic vapor phase epitaxy (MOVPE) with trimethylaluminum (TMAl), trimethylgallium (TMGa) and ammonia (NH3) precursors. Nitriding (optional) and nucleation, at 1200 ° C, produces an island of nano-cones grown at 1150 ° C, silane (SiH4) for n-type doping. The researchers estimate that the growth surface temperature is about 100 ° C lower than the thermocouple reading.
The resulting nanocone has a hexagonal base. From the crystal structure, the side faces are {10-12} planes, and the substrate is inclined at an angle of about 43°. “Because of the low amount of NH3 used for nanopyramid growth, the growth rate along the <10-12> direction is slow, resulting in the formation of {10-12} sides,†the team explained. Some nanocones have unclear sides - this is due to the formation of multiple AlGaN seed crystals that are subsequently merged on the nucleation layer.
Photoresponse from electron beam excitation - Cathodoluminescence (CL) - produces a peak at 366 nm (Figure 2). There is also a very strong defect-related luminescence near 550 nm. Although about 2.8% aluminum was present during the growth process, the peak at 366 nm appeared to be from the edge-emitting emission of pure GaN. The researchers commented: "This suggests that suboptimal growth conditions result in a decrease in material quality, broadband emission and strong defect luminescence also indicate that this may be due to the flow of low amounts of NH3 and high amounts of silane."
Figure 2: (a) and (b) room temperature CL spectra obtained from the samples in Figures 1 (b) and (c) and the single pyramid from the overgrown pyramid. (c) and (d) SEM images and panchromatic CL intensity maps of the same pyramid. (e) Pyramid array samples of top view SEM overgrowth in Figure 1(d). Illustration: Magnified sample; scale 2 μm.
The problem of low aluminum incorporation is addressed by "improving growth techniques. For example, pulse growth techniques or using vertical flow MOVPE reactors."
The researchers added the MOVPE AlGaN / GaN / AlGAN heterostructure to the nanocone, which grew 20% of the aluminum in the gas phase. According to transmission electron microscopy (TEM), the total overgrowth thickness is about 300 nm. The GaN deposition lasted for 1 minute while the AlGaN layer was grown for 15 minutes.
According to the team, CL from overgrown materials shows peaks around 363 nm, and many peaks on longer wavelength shoulders may be "whispering wall patterns." In this case, no peaks related to defects were seen.
The researchers commented: "This superior optical quality of the overgrowth pyramid may be due to the use of high NH3 flow resulting in a very regular hexagonal shape that gives it a smooth side."
The TEM of the cross section shows the presence of a cavity at the base of the nanocone, indicating that the growth at the edge of the graphene of the pore is nucleated.
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